I'm not personally a fan of using icons in form fields, but I think it should be pointed out that they are not always just a purely stylistic choice.
One of the reasons icons are increasingly used for email/password forms in particular is: browsers and plugins (like Lastpass) are increasingly insistent on autofilling these fields. And autofill behavior is increasingly difficult or undesirable to block client-side.
So the placeholder text (e.g. 'Enter email') inside the input fields often gets automatically overwritten, and when a user goes to sign in she may be presented with an auto-filled dialog that looks like this:
In this situation, designers have a choice to: (1) leave the fields without labels and assume the user understands what the unlabeled fields are for; (2) place a traditional text label ('email', 'password') somewhere on the form; or (3) use an icon which will be ignored if the field is unfilled (since there is a placeholder), but serve as a subtle and space-saving indicator to the field's purpose if it is filled.
Personally I do not like the use of icons, but I do think it's important to note that the choice to use them is not an act of stylistic hubris or design whimsy, but can actually be informed by good, solid UX thinking and observation.